1. Field of the Invention
The present invention relates generally to a double-side printing apparatus for printing on double sides of a recording medium and, more particularly, to a double-side printing apparatus in which a plurality of electrophotographic recording units are disposed.
2. Related Background Art
A printing apparatus has been widely utilized as an output apparatus of a computer. An electrophotographic apparatus capable of printing on an ordinary sheet of paper has been utilized as the printing apparatus. In response to a demand for saving natural resources in recent years, a double-side printing apparatus for printing on double sides of the sheet has been required. Then, an apparatus provided with both a printing mechanism for printing on the right side of the recording medium and a printing mechanism for printing on the reverse side of the recording medium, is required for increasing printing speed.
FIG. 7 is an explanatory view showing the prior art.
The double-side printing apparatus includes an electrophotographic printing unit (a second image forming unit) 91 for printing on the right side of a sheet of recording paper P, and an electrophotographic printing unit (a first image forming unit) 92 for printing on the reverse side of the sheet P. The sheet P is classified as continuous paper perforated to delimit each page. The reverse side printing unit 92 has a photosensitive drum. The photosensitive drum is charged by a pre-charger and thereafter exposed to a light image by an exposing unit. An electrostatic latent image corresponding to the light image is thereby formed on the photosensitive drum. The latent image on the photosensitive drum is developed by a developing unit. Then, the developed image on the photosensitive drum is transferred onto the sheet P by a transferring unit. Thus, the image is printed on the reverse side of the sheet P.
The printing on the right side of the sheet P is likewise performed. To be specific, the right side printing unit 91 has a photosensitive drum. The photosensitive drum is charged by a pre-charger and thereafter exposed to a light image by an exposing unit. An electrostatic latent image corresponding to the light image is thereby formed on the photosensitive drum. The latent image on the photosensitive drum is developed by a developing unit. Subsequently, the developed image on the photosensitive drum is transferred onto the sheet P by a transferring unit. Thus, the image is printed on the right side of the sheet P.
Next, a toner image on the reverse side of the recording sheet P is fixed by a first fixing unit 93. Then, a toner image on the right side of the recording sheet P is fixed by a second fixing unit 94. Thus, according to the described double-side printing process, after the toner image has been formed on the right side, the toner image is formed on the reverse side. And then a fixing process is performed, thereby the double-side printing apparatus can be downsized. This type of double-side printing apparatus for printing on the continuous paper is disclosed in Japanese Patent Application Laid-Open Publication Nos.7-77851 and 8-211664.
There arise, however, the following problems inherent in the prior art.
First, if flash fixing units for fixing by a flash light are used as the fixing units 93, 94, the image can be fixed in a non-contact manner onto the sheet. Therefore, even when unfixed images are fixed by continuous feeding, it never happens that the unfixed images are disturbed. The flash has, however, a high intensity, and hence leaked flash beams strike upon the photosensitive drum of the electrophotographic printing unit, resulting in such a problem that the photosensitive drum might be deteriorated. Especially, the flash beams from the first fixing unit 93 impinge on the photo sensitive drum of the second electrophotographic printing unit 91 through the sheet P, and the problem is also the deterioration of this photosensitive drum.
Second, it is necessary for stabilizing the carry of the sheet that a guide member is provided between the first fixing unit and the second electrophotographic printing unit 91. The guide member is, however, brought into contact with the unfixed image on the sheet, and consequently the toner image is offset by the guide member, with the result that the disturbance of the unfixed image on the sheet might occur as another problem.